October 1995 Bulletin

Many unknowns in ACL repair

by
Louis J. Soslowsky, PhD Steven A. Goldstein, PhD

Lou Soslowsky, PhD, is assistant professor of orthopaedic surgery and mechanical engineering/applied mechanics, University of Michigan.

Steve Goldstein, PhD, is professor of orthopaedic surgery and mechanical engineering/applied mechanics, assistant dean for research and graduate studies, Medical School and director of the Orthopaedic Research Laboratories, University of Michigan.

It is estimated that more than 100,000 anterior cruciate ligament (ACL) injuries occur every year in the United States, resulting in significant constraints on activities and potentially accelerating degenerative joint conditions. While many of these injuries require repair and/or reconstructive procedures, the optimal choice of techniques, devices, or repair strategies remains unresolved.

Perhaps the first dilemma faced by the surgeon is the choice of material to use as a reconstructive graft. While a variety of synthetic materials have been used for grafts, these materials have not become particularly popular for a variety of reasons. The inherent mechanical properties of synthetic grafts do not closely resemble those of the normal ligament and these artificial ligaments do not have the capability to repair and remodel.

Alternatively, biologic grafts, both autografts and allografts, are frequently used. A variety of autografts such as a portion of a bone-patellar tendon-bone complex, the iliotibial tract, semitendinosus tendon, and gracilis tendon have been used to replace the ACL. The bone-PT-bone graft is the most popular and it has been most extensively investigated. In brief, the patellar tendon graft complex exhibits approximately 30-50 percent of the ultimate load of the ACL complex at one to two years postsurgery in animal studies.

Allografts have been considered for many patients due to the availability of tissue and the potential morbidity associated with autograft procedures. The tissues are usually preserved either by deep-freezing or by freeze-drying. While sterilization is necessary, it has been demonstrated to adversely affect the mechanical properties of the tissues. In vivo animal studies using allografts have demonstrated similar findings to those of autografts, i.e., decreased properties relative to controls, and thus there exists no truly ideal ligament graft replacement.

Many factors

The function of any graft depends on many factors including: graft placement, graft fixation, initial graft tension, and possibly graft augmentation. For graft placement, the critical goal is to place the graft such that large changes in graft length, and, therefore, large changes in graft tension, are avoided. It has been shown in the normal ACL, that no single position exists which is truly isometric due to the complexity of the ligament fiber bundles. However, locating the most consistent "anatomic placement" is desired since it is this position which most closely allows restoration of normal joint kinematics.

Graft fixation methods include buttons, augmentation screw and post, staples, and interference screws, with the latter being the most popular for ACL reconstructions. Choosing the ideal initial graft tension is difficult since slight initial tension differences can create drastically altered joint kinematics, graft load, and graft deformation during motion. An initial tension that is overly large will cause large graft forces with a small amount of motion thereby placing the reconstruction in a compromised state. A tension that is too small will not provide sufficient joint stability and might result in excessive joint translations. In any case, it should be noted that the graft tension will be the largest at the moment it is fixed and will decrease significantly over time due to its viscoelastic nature.

Need study

In order to provide for initial stability of the graft, ligament augmentation devices are sometimes employed. While it is clear that an augmentation device can shield a ligament graft from large loads, it is unclear whether this is an advantageous strategy. In order for the graft to biologically remodel appropriately, it is believed that the graft should experience a certain range of load or deformation. Deviation from these conditions (due to augmentation) might adversely affect incorporation of the graft. These issues remain under current investigation.

There are several methods to assess the result of the ligament reconstruction such as biomechanical tests in the laboratory, subjective or objective measures, evaluation of activity levels of the patient, functional/performance tests, and outcome studies. Laboratory biomechanical tests have been performed on the various graft materials as well as on grafts utilized in animal models. The most critical evaluative variables include graft stiffness, response to cyclic loading, ultimate strength, and joint kinematics. These properties are particularly important to evaluate as a function of time in animal models. Subjective assessment of reconstructive procedures are provided by patients, based on their belief or feeling regarding the success of the reconstruction.

A variety of evaluation forms can be used to categorize this assessment. Objective patient assessments are obtained from clinical measurements such as anterior tibial translation, total AP translation, posterior sag, and medial and lateral joint opening. While these measures are generally categorized as objective, it should be recognized that some subjectivity also can exist when observer variability is considered. Measuring the activity level of the patient some time after surgery is often an important indicator of the success of the reconstruction. While the ability to return to one's job or sport is an extremely important consideration, this measure is difficult to quantify and/or compare between series of patients.

Specific functional or performance tests have been described that assess the ability of the patient to perform a certain movement such as the one-legged hop. While the environment for these types of tests is more controlled than the more general category of "return to sport," for example, quantitative comparisons to appropriate controls are often difficult. Outcome studies have received much attention recently and may be an appropriate method to test the relative success of a ligament reconstruction. This approach uses rigorous scientific methodology to test, for example, whether a specific ligament reconstruction method is favorable to another.

While much is known about the pros and cons of various ligament reconstructions, even more remains unknown at this time. Current and future research will attempt to elucidate the advantages and appropriate uses of the various methods so that the surgeon can assess the best method to be used for any specific patient.

References

Woo SLY, An KN, Arnoczky SP, Wayne JS, Fithian DC, Myers BS: Anatomy, Biology, and Biomechanics of Tendon, Ligament, and Meniscus, in Orthopaedic Basic Science, Simon SR (ed), AAOS, 1994, pp. 45-87.

Fu FH, Hamer CD, Johnson DL, Miller MD, Woo SLY: Biomechanics of Knee Ligaments: Basic Concepts and Clinical Applications, in Instructional Course Lectures, Vol. 43, Schafer, M. (ed), AAOS,1994, pp. 137-148.

Wojtys EM (ed): The ACL Deficient Knee, 1994, AAOS.


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